Devices are known with a horizontally mounted cylindrical container, a hollow shaft with glue feed means therein, and mixing tools and glue stirring elements. Similar devices are known, for example, from U.S. Pat. No. 3,734,471. They have been found to be very valuable in practice for coating wood chips, especially for coating wood chip mixtures consisting of coarse and medium chips and sawdust. The glue is uniformly distributed over the chips in these known devices by a forced mixing effect in a compact, i.e., relatively dense, mix ring which forms on the inside wall of the mixing container. The mix ring is the ring or cylinder of material being mixed which is held against the outside of the drum by centrifugal force. It was clear to experts looking at these mixers that in order to achieve a uniform coating of the individual chips and particularly to avoid considerable differences in residence times of the individual chips in the mixing container, the passage of air through the mixer should be avoided as much as possible.
In an attempt to use these known ring mixers for coating fibers, it was found that the fibers lump together considerably and jam the mixing container. Hence, satisfactory homogeneous coating of the fibers could not be achieved.
The coating of fibers, especially wood fibers, is very important to the manufacture of so-called wood fiber panels. Such fiber panels, in contrast to normal chip board which has only one smooth surface, can be worked on both the surface and side edges and exhibits a good surface quality at those locations as well.
German Auslegeschrift 1,048,013 discloses an impeller or agitator mixer for the coating of wood chips with dusty components, in which the glue is sprayed into the mixing container through nozzles provided in the upper surface of the horizontally mounted cylindrical mixing container. In this device an air stream is blown axially through the mixing container in order to reduce the residence time of the dusty chip particles relative to the residence time of the coarser chips so as to largely reduce the relatively excessive coating of these dusty wood chip particles. The problem of avoiding relatively excessive coating of dusty particles does not occur in the coating of pure fibers, however.
It is know from German Offenlegungsschrift 1,632,450 to coat wood chips agitated in an air stream in a mixing tube in which glue spray nozzles are mounted.